Structural elucidation of the predominant motifs of the major cell wall arabinogalactan antigens from the borderline species Tsukamurella paurometabolum and Mycobacterium fallax

Abstract

Tsukamurella paurometabolum and Mycobacterium fallax are members of the suprageneric actinomycete group Corynebacterineae that possesses a cell wall skeleton composed of a peptidoglycan to which an arabinogalactan is covalently attached. This polysaccharide is further modified by esterification with C₆₀–C₈₀ mycolic acid residues in mycobacteria and T. paurometabolum. However, M. fallax and T. paurometabolum produce polyenoic (up to six double bonds) mycolic acids whereas the most common type of mycobacterial mycolates, called α-mycolates, are mono- and di-enoic or -cyclopropanated mycolic acids. To determine whether this difference also applied to the structures of cell wall arabinogalactans, competitive inhibition experiments using antibodies raised against the cell wall from Mycobacterium bovis and the arabinogalactans from T. paurometabolum and M. fallax were performed. They demonstrated the structural identity between the polysaccharide of M. fallax and those of mycobacteria and showed a strong similarity between the latter polysaccharides and that of T. paurometabolum. Structural analyses of the per-O-alkylated alditol fragments derived from the polysaccharides by gas chromatography–mass spectrometry (GC-MS) and ¹³C nuclear magnetic resonance (NMR) spectroscopy of the intact solubilized polysaccharides demonstrated that the polysaccharides from the two species analyzed contained all the major structural features previously characterized in mycobacterial arabinogalactans. These include (1) the homogalactan of alterning 5-linked galactofuranosyl (Galf) and 6-linked Galf residues, (2) a linear 5-linked arabino furanosyl (Araf), (3) a β-Araf-(1→2)-α-Araf disaccharide branched on both position 3 and position 5 of an α-Araf unit, and (4) a 5-linked-α-Araf unit branched on both position 3 and position 5 of an α-Araf residue. The polysaccharide from T. paurometabolum possesses additional structural domains composed of a terminal (t) Araf directly linked to either a 5-linked-α-Araf or to both position 3 and position 5 of a 3,5-linked α-Araf unit. Both the remarkable similarity of arabinogalactans from Corynebacterineae and their genus- and/or species-specificities are reflected in their ¹³C NMR spectra that may be used as a valuable help in the identification of members of the actinomycete group.